Spin reorientation in Na-doped BaFe$_2$As$_2$ studied by neutron diffraction

TL;DR

This study investigates the magnetic phase transitions in Na-doped BaFe$_2$As$_2$ using neutron diffraction, revealing two successive magnetic transitions including a spin reorientation, highlighting complex magnetic behavior influenced by spin-orbit coupling.

Contribution

It provides the first detailed neutron diffraction analysis of magnetic ordering and spin reorientation in Na-doped BaFe$_2$As$_2$, showing the importance of spin-orbit coupling.

Findings

Two magnetic transitions at 70 K and 46 K in Na-doped BaFe$_2$As$_2$

Identification of a spin reorientation at low temperature

Magnetic moments align along the c-axis at low temperatures

Abstract

We have studied the magnetic ordering in Na doped BaFe$_2$As$_2$ by unpolarized and polarized neutron diffraction using single crystals. Unlike previously studied FeAs-based compounds that magnetically order, Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ exhibits two successive magnetic transitions: For x=0.35 upon cooling magnetic order occurs at $\sim$70\ K with in-plane magnetic moments being arranged as in pure or Ni, Co and K-doped BaFe$_2$As$_2$ samples. At a temperature of $\sim$46\ K a second phase transition occurs, which the single-crystal neutron diffraction experiments can unambiguously identify as a spin reorientation. At low temperatures, the ordered magnetic moments in Ba$_{0.65}$Na$_{0.35}$Fe$_2$As$_2$ point along the $c$ direction. Magnetic correlations in these materials cannot be considered as Ising like, and spin-orbit coupling must be included in a quantitative theory.